) Emission ratio A(N Au

Exciton-Plasmon Interactions and Fano Resonances

in Nanostructures

Alexander Govorov

Department of Physics and Astronomy, Ohio University, Athens, Ohio, USA

Supported by:

NSF, Air Force Research Office, A. v. Humboldt Foundation,

BNNT Institute at Ohio U.

Collaborators:

Pedro Hernandez, Ohio U.

Fan Zhiyuan, Ohio U.

Wei Zhang, Ohio U. (now in China)

Nanoparticles as building blocks Interaction between nanocrystals

Exciton + plasmon + photons

Linker (polymer)

liquid

Quantum emitter

Amplification or suppression of PL

++ + + -

-- -

exciton

plasmon Au

CdTe

++ + + -

-- -

Au

CdTe

p laser

emission exc

p

E























Incoherent exciton-plasmon interaction

2

, int

2 ˆ 2

( ) | 0; ; 0 | ( ) Im[ ( )]

non rad metal exc exc f exc

f T

f V exc

 _   _



2 2

2

)

(

metal t no

actual t

E E E

P   

exc1

exc0

0

Metal NP

Semiconductor NP

++ ++ -

-- -

Au

CdTe

Book: Joseph R. Lakowicz

Principles of Fluorescence Spectroscopy Energy transfer (FRET) rate:

 

0

0 0

( ) ( )

exc

rad exc non rad transfer exc l

dn P n P I

dt      

   

Slocik, J.M.; Govorov, A.O.;

and Naik, R.R.,

Supramolecular Chemistry, 2006.

N Au-NP -- CdSe-NP

Number of Au NPs

N_Au=1 N_Au=2 N_Au=4 N_Au=6

400 500 600 700 800 900 1000 0.8

0.9 1.0 1.1 1.2

N_Au=4,2

N_Au=1 N_Au=6

Enahncement factor

Wavelength,  (nm) ^{0 1 2 3 4 5 6 7 8}

0.0 0.2 0.4 0.6 0.8 1.0 1.2

theory

exper

Number of Au NPs, N_Au

_emiss=610 nm

_laser=400 nm

Emission ratio A(N Au)

0 0

( )

(0)

emiss Au NPs

emiss transfer

I N

A I N



 





 

Emission ratio:

Govorov, et al., Nano Lett., 2006

Coherent interactions.

Fano effect

1

2 k

V

v w

2 2 2

0

2 2 2

2

0

( )

2

E v q

Abs w

w q V w

v







   

 

 

 

-40 -20 20 40

0.05 0.1 0.15 0.2 0.25 0.3

    

Absorption

Detuning,

X-ray absorption spectrum of He

-40 -20 20 40

0.05 0.1 0.15 0.2 0.25 0.3

Fano effect in He

2 1s¹ k

V

v

w

X-ray absorption spectrum of He

Fano lineshapes

1

2 k

V

v w

2

2 2

12 2

0

( q )

Abs

w q V w

v



   



  

 

 

 

 

-10 -5 0 5 10

0.0 0.5 1.0 1.5

-10 -5 0 5 10

0.0 0.5 1.0 1.5

Absorpt ion

-10 -5 0 5 10

0.0 0.5 1.0 1.5

q=0.01 q=2

Detuning

q=100

,     

k +

+ + + -

-- -

exciton plasmons MNP

SQD

Coulomb interaction













   

  

k

Coul k

k k

k

a a U c c a U c c a

c c E c

c E

H ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ

2 1

* 1

2 2

2 2 1

1 1

0



1 2

exciton plasmon states

ground state

Colloidal nanocrystals

Two paths for excitation of plasmon 

interference effect (Fano effect)

Optical absorption

k

1 2

exciton plasmon states

ground state +

+ + + -

-- -

exciton plasmons MNP

SQD

Coulomb interaction

Fano effect

1

2 k

V

v w

-40 -20 20 40

0.05 0.1 0.15 0.2 0.25 0.3





  

absorption

2 2 2

0

2 2 2

2

0

( )

( )

2

E v q

Abs w

w q V w

v

 





   

 

 

 

Light

  t t

V

t V H

H

H i H

t

a c c U

a c c U

a a c

c E c

c E H

opt

opt

k Coul

k k

Coul k

k k

k







 



cos )

ˆ (

) ˆ (

ˆ ˆ

ˆ ˆ ˆ )

ˆ ˆ ˆ

ˆ (

ˆ ˆ ˆ ˆ

ˆ ˆ ˆ

ˆ ˆ

ˆ ˆ

ˆ ˆ

0 0

2 1

* 1

2 2

2 2 1

1 1 0

E r 



















 



















Strong de-coherence in the metal NP (fast relaxation of plasmon)

Electromagnetic enhancement due to the plasmon

+ + + + -

-- -

exciton plasmons MNP

SQD

W. Zhang, A. Govorov, et al., PRB 2008.

Dipole limit:

tot MNP SQD

Q  Q  Q

12

10 1

metal NP semiconductor NP

metal NP

R nm

meV

  



 

100nm

2.0 2.2 2.4 2.6

0 2 4 6 8 10 12 14

Photon energy ( ^eV )

Ex ti nc ti on,  ( 10

cm

M

)

W. Zhang, A. Govorov, et al., PRB 2008.

In the regime of strong de-coherence:

Strong anti-resonances in absorption and light scattering

Weak lines become visible as anti-resonances

Multipoles are important

NP tor semiconduc NP

metal



 

CdTe-Au complex

12nm

tot MNP SQD

Q  Q  Q

12 1

metal NP 10 meV

R nm

 



Nonlinear Fano effect

1

2 k

V

v w

Strong light field

Transition 1 2 is partially saturated

 

1 2 22 11

Absorption

   

Experiments in Munich

A dot with weak tunnel coupling to a continuum Fano factor

Narrow exciton resonances

Self-assembled quantum dots at low temperatures

InAs

Quantum well

 1 q

AlGaAs

-0.5 0.0 0.5 1.0

-50 0 50 -50 0 50 -50 0 50 -50 0 50 -50 0 50 -50 0 50

-0.5 0.0 0.5

1.0 ^{0.41µeV 2.1µeV 6.9 µeV 9.5 µeV 16 µeV} 0.33 nW 8.3 nW 90 nW 168 nW

22 µeV 481 nW 935 nW



- 

Normalized extinction

Martin Kroner, Khaled Karrai, at al., experiments

Theory

M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A.

Badolato, P. M. Petroff, W. Zhang, R. Barbour, B. D. Gerardot, R. J.

Warburton & K. Karrai, Nature, 2008.

Normalized differential absorption

power

Nonlinear Fano effect

Low power High power

At low power, the natural broadening prohibits observation of weak processes

At high power, we can observe weak coherent processes The discrete resonance is saturated

Transitions to the continuum become more important

power

absorption

Discrete

continuum

laser

 1

q

metal NP molecule

  

Light

0 0

metal NP

molecule

CD CD





Optical chirality (circular dichroism)

Circular dichroism spectroscopy

0

log

lcp lcp

lcp

lcp rcp

CD lcp rcp

A I L c

I

A A A



   

   

  

   

0

( )

lcp rcp

I I

k 

Chiral objects:

No mirror symmetry planes Example: Helix

1

2

molecule density of states of NP

photons



Coulomb

NP

dye

 ^

m 

R 

a

k 

 ^

150 200 250 300 350 400 450 500 -40

-20 0 20 40 60 80

CD,  (cm-1 M-1 )

Wavelength (nm)

plasmon-induced

*,

  

*, ||

 

*, ||

n

150 200 250 300 350

0 2000 4000 6000 8000

Wavelength (nm) Extinction (cm-1 M-1 )

Ag

*,

  

*, ||

n

*, ||

 

-helix Ag NP

W. Moffitt, J. Phys. Chem. 25, 467 (1956).

Chromophore excitons are delocalized in a helix structure

 ^ z

 ^ x

 ^ y

Govorov, A.O.; Fan, Z.; Hernandez, P.; Slocik, J.M; Naik, R.R., Nano Letters, 2010.

Purely plasmonic CD

Z. Fan, A. O. Govorov, Nano Letters 2010.

The exciton-plasmon interactions

Linear and nonlinear interference effects Plasmon-induced CD

Conclusions

Low power High power

Ag

150 200 250 300 350 400 450 500 -40

-20 0 20 40 60 80

CD, (cm-1 M-1 )

Wavelength (nm) plasmon-induced